Plasmas have become convenient sources of energetic ions and activated atoms which can be employed in a variety of semiconductor device fabrication processes including surface treatments, depositions, and etching processes. For example, to deposit materials onto a semiconductor wafer using a sputter deposition process, a plasma is produced in the vicinity of a sputter target material which is negatively biased. Ions created adjacent the target impact the surface of the target to dislodge, i.e., "sputter" material from the target. The sputtered materials are then transported and deposited on the surface of the semiconductor wafer.
Sputtered material has a tendency to travel in straight line paths, from the target towards the substrate, at angles which are usually oblique to the surface of the substrate. As a consequence, materials deposited in etched openings, including trenches and holes of semiconductor devices having openings with a high depth to width aspect ratio, may not adequately coat the walls of the openings, particularly the bottom walls. If a large amount of material is being deposited, the deposited material can bridge over the opening, causing undesirable cavities in the deposition layer. To prevent such cavities, sputtered material can be redirected into more vertical paths between the target and the substrate by negatively biasing (or self-biasing) the substrate and positioning appropriate vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered by a low density plasma often has an ionization degree of less than 10 percent, which is usually insufficient to avoid the formation of an excessive number of cavities. Accordingly, it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease the formation of cavities in the deposition layer. As used herein, the term "dense plasma" is intended to refer to one that has a high electron and ion density, typically in the range of 10.sup.11 -10.sup.13 ions/cm.sup.3.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil surrounding the plasma induces currents in the plasma. These currents heat the conducting plasma so that it is sustained in a steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance-matching network, such that the coil acts as the first windings of a transformer. The plasma acts as a single turn second winding of a transformer.
Although ionizing the deposition material facilitates deposition of material into high aspect ratio channels and vias, many sputtered contact metals have a tendency to deposit more thickly in the center of the wafer as compared to the edges. This "center thick" deposition profile is undesirable in many applications where a uniform deposition thickness is needed.
As described in copending application Ser. No. 08/680,335, entitled "Coils for Generating a Plasma and for Sputtering," filed Jul. 10, 1996 and assigned to the assignee of the present application, it has been recognized that the coil itself may provide a source of sputtered material to supplement the deposition material sputtered from the primary target of the chamber. Application of an RF signal to the coil can cause the coil to develop a negative bias, which attracts positive ions to impact the coil and sputter material from the coil. Because the material sputtered from the coil tends to deposit more thickly at the periphery of the wafer, the center thick tendency for material sputtered from the primary target can be compensated by the edge thick tendency for material sputtered from the coil. As a result, uniformity can be improved when sputtering from a target and coil fabricated from materials such as aluminum and titanium.
Materials other than aluminum and titanium are sometimes deposited as part of interconnect structures. For example, a tungsten plug may be used to connect different layers in a device. In addition, wiring lines often utilize barrier or adhesion films between the wiring line and the underlying layer. Certain high melting point metals such as tungsten and tantalum are sometimes used as barrier films. These metals tend to be more brittle than aluminum and titanium. As a result, it may be difficult or prohibitively expensive to manufacture complicated or large structures from these high melting point metals.
U.S. Pat. No. 5,178,739 notes the problem of contamination due to material flaking off of a coil during sputtering and becoming deposited on the workpiece. The '739 patent proposes to fabricate the coil out of, or coat it with, the material being deposited. The '739 patent does not offer any method of creating such a coating, however. U.S. Pat. No. 5,707,498 recognizes the same problem, and proposes a pasting step to coat a coil with the material being sputtered. Such a pasting step is performed to coat the coil with the target material prior to sputtering the target material onto the workpiece. In one example, the pasting step may be accomplished by sputtering a titanium target prior to inserting the workpiece into the chamber. The pasting process, however, deposits a layer on the coil in a relatively slow manner, and depending on the thickness desired, a significant amount of time may be required to form the layer. The additional time and process step required for carrying out such a pasting step undesirably lowers the system throughput.